There have been few major improvements in conventional lighting (i.e. incandescent, halogen, and fluorescent lamps) over the past 20 years. However, in the case of light emitting diodes (“LEDs”), operating efficiencies have been improved to the point where they are replacing incandescent and halogen lamps in traditional mono-chrome lighting applications, such as traffic lights and automotive taillights. This is due in part to the fact that LEDs have many advantages over conventional light sources that include long life, ruggedness, low power consumption, and small size. LEDs are monochromatic light sources, and are currently available in various colors from UV-blue to green, yellow, and red. Furthermore, due to LEDs' narrow-band emission characteristics, a white color LED can only be produced by: 1) arranging individual red, green, and blue (R, G, B) LEDs closely together and then diffusing and mixing the light emitted by them; or 2) combining a short-wave UV or blue LED with broadband fluorescent compounds that convert part or all of the LED light into longer wavelengths.
When creating a white LED using the first approach described above, several problems arise due to the fact that the R, G, B light emitting devices are made of different semiconductor materials, which require different operating voltages and, therefore, complex driving circuitry. Other disadvantages include low color-rendering characteristic of the resulting white light due to the monochromatic nature of the R, G, B LED emissions, and the different emission temperature dependences of the different semiconductor materials.
The second approach for producing white light from LEDs is in general more preferred, since it only requires a single type of LED (either UV or blue) coated with one or more fluorescent materials, thereby making the overall construct of a white light producing LED more compact, simpler in construction, and lower in cost versus the former alternative. Furthermore, the broadband light emission provided by most fluorescent materials or phosphors allows the possibility of high color-rendering white light.
A recent breakthrough in the efficiency of UV/blue LEDs has resulted in phosphor-coated blue LEDs becoming a serious contender for replacing conventional incandescent bulbs used in the current illumination and display backlighting applications. Most of the current commercially-available devices work by converting a portion of the blue LED emission to yellow. In such a situation, some of the blue light from the LED is transmitted through the phosphor and mixed with the yellow phosphor emission, thereby resulting in a perceived white light. Many workers have delved in the field of phosphors as evidenced by the following US patents, which are expressly incorporated by reference.
U.S. Pat. No. 4,512,911 discloses a rare earth element activated complex halide phosphor represented by the formula:BaF2.aBaX2.bMgF2.cBeF2.dMeIIF2:eLnwherein X is at least one halogen selected from the group consisting of chlorine, bromine and iodine; MeII is at least one divalent metal selected from the group consisting of: calcium and strontium; Ln is at least one rare earth element selected from the group consisting of: divalent europium (Eu2+), cerium (Ce3+) and terbium (Tb3+), and a is in the range between 0.90 and 1.05, b is in the range of 0 to 1.2; c is in the range of between 0 and 1.2, and d is defined by the sum of c+d being in the range of between 0 and 1.2, and BeF2 is present in an amount sufficient to effect a phosphor exhibiting a higher luminance than said phosphor absent BeF2 when stimulated by light of a wavelength ranging from 450 to 800 nm after exposure to X-rays.
U.S. Pat. No. 4,661,419 teaches a cerium activated rare earth halophosphate phosphor having the formula:LnPO4.aLnX3:xCe3+in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; X is at least one halogen selected from the group consisting of F, Cl, Br and I; and a and x are numbers satisfying the conditions of 0.1<a<10.0 and 0<x<0.2, respectively and exhibiting a higher stimulated emission upon excitation with a He-Ne laser of a wavelength 632.8 nm after exposure to X-rays at 80 KVp, than the phosphor wherein a is less than 0.1.
U.S. Pat. No. 5,140,604 provides mixed single-phase strontium and lanthanide oxide with a magnetolead type crystalline structure having the formula (I):SrxLn1y1Ln2y2Ln3y3MzAaBbO19-k  (I)in which Ln1 represents at least one trivalent element selected from lanthanum, gadolinium and yttrium; Ln2 represents at least one trivalent element selected from neodymium, praseodymium, erbium, holmium and thulium; Ln3 represents an element selected from bivalent europium or trivalent cerium with retention of electric neutrality by virtue of oxygen holes; M represents at least one bivalent metal selected from magnesium, manganese, and zinc; A represents at least one trivalent metal selected from aluminium and gallium; B represents at least one trivalent transition metal selected from chromium and titanium; x, y1, y2, y3, z, a, b and k represent numbers so that 0<x<1, 0<y1<1, 0<y2<1, 0<y3<1, 0<z<1, 10.5<a<12, 0<b<0.5 and 0<k<1 provided that 0<x+y1+y2+y3<1 and that 11<z+a+b<12.
U.S. Pat. No. 5,198,679 teaches a divalent europium activated alkaline earth metal halide phosphor having the formula:MIIX2.aMIIX′2.bSiO:xEu2+
in which MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X′ is at least one halogen selected from the group consisting of Cl, Br and I, and X is not the same as X′; a and x are numbers satisfying the conditions of 0.1<a<10.0 and 0<x<0.2, respectively; and b is a number satisfying the condition of 0<b<3×10−2.
U.S. Pat. No. 5,602,445 teaches a bright, short wavelength blue-violet phosphor for electro luminescent displays comprising an alkaline-based halide as a host material and a rare earth as a dopant. The host alkaline chloride can be chosen from the group II alkaline elements, particularly SrCl2 or CaCl2, which, with a europium or cerium rare earth dopant, electroluminesces at a peak wavelength of 404 and 367 nanometers respectively. The resulting emissions have CIE chromaticity coordinates which lie at the boundary of the visible range for the human eye thereby allowing a greater range of colors for full color flat panel electroluminescent displays.
U.S. Pat. No. 5,648,181 describes an inorganic thin film electroluminescent device, comprising an inorganic light emission layer, a pair of electrodes and a pair of insulating layers, at least one of the electrodes being optically transparent, the light emission layer being positioned between the pair of insulating layers, each insulating layer being formed on an opposite side of the light emission layer, the pair of insulating layers being positioned between a light emission layer and the pair of electrodes, the light emission layer consisting essentially of inorganic material comprising a matrix of lanthanum fluoride doped with at least one member selected from the group consisting of: rare earth element metals and compounds thereof.
U.S. Pat. No. 5,698,857 teaches a radiographic phosphor screen comprising a support and, coated on the support, at least one layer forming a luminescent portion and an overcoat layer, the luminescent portion and overcoat layer including a binder that is transparent to X-radiation and emitted light and said luminescent portion including phosphor particles in a weight ratio of phosphor particles to binder of 7:1 to 25:1. The phosphor comprises oxygen and a combination of species characterized by the relationship:(Ba1-qMq)(Hf1-z-eZrzMge):yTwherein M is selected from the group consisting of Ca and Sr and combinations thereof; T is Cu; q is from 0 to 0.15; z is from 0 to 1; e is from 0 to 0.10; z+e is from 0 to 1; and y is from 1×10−6 to 0.02.
U.S. Pat. No. 5,998,925 provides a light-emitting device, comprising a light emitting component and a phosphor capable of absorbing a part of light emitted by the light emitting component and emitting light of wavelength different from that of the absorbed light; wherein the light-emitting component comprises a nitride compound semiconductor represented by the formula: IniGajAlkN where 0<i, 0<j, 0<k and i+j+k=1 and the phosphor contains a garnet fluorescent material comprising: 1) at least one element selected from the group consisting of Y, Lu, Se, La, Gd and Sm; and 2) at least one element selected from the group consisting of Al, Ga and In, and being activated with cerium. One inorganic phosphor used in commercial white LEDs is the cerium-doped yttrium aluminum garnet Y3Al5O12:Ce (YAG:Ce) and its derivative phosphors described in this patent, which is regarded by many in the field as being the standard inorganic phosphor used in commercial white LEDs as of this writing.
U.S. Pat. No. 6,066,861 teaches a wavelength-converting casting composition, for converting a wavelength of ultraviolet, blue or green light emitted by an electroluminescent component, comprising: a) a transparent epoxy casting resin; b) an inorganic luminous substance pigment powder dispersed in the transparent epoxy resin, the pigment powder comprising luminous substance pigments from a phosphorus group having the general formula:A3B5X12:M,where A is an element selected from the group consisting of Y, Ca, Sr; B is an element selected from the group consisting of Al, Ga, Si; X is an element selected from the group consisting of O and S; and M is an element selected from the group consisting of Ce and Tb. The luminous substance pigments have grain sizes <20 μm and a mean grain diameter d50<5 μm.
U.S. Pat. No. 6,153,971 describes a method for illuminating an object that allows categorical color perception of at least red, green, blue, yellow and white on the surface of the illuminated object, the method comprising: illuminating the object with light consisting essentially of the combination of light of two major wavelength bands, in which: the first wavelength band is from 530 to 580 nm; and the second wavelength band is from 600 to 650 nm.
U.S. Pat. No. 6,255,670 teaches a composition of matter comprising Ba2(Mg,Zn)Si2O7:Eu2+, as well as a composition of matter comprising: (Ba1-X-Y−Z,CaX,SrY,EuZ)2(Mg1-w,Znw)Si2O7, wherein X+Y+Z1; Z>0; and 0.05<W<0.50.